如何从组合框中获取选定的值 [英] How to get selected value from combo box

问题描述

我正在尝试从数据库实现自动完成组合框，如下面的代码所示。我的目标是获得所选项目的价值。虽然它返回了所需的值但是错误对象引用未设置为对象的实例。请帮我。谢谢。

 私人  void  Form_Load（ object  sender，EventArgs e）
 {
 cboTests.AutoCompleteSource = AutoCompleteSource.CustomSource; 
 cboTests.AutoCompleteMode = AutoCompleteMode.SuggestAppend; 
 LoadTest（）; 
 
} 
 
  private   void  LoadTest（） 
 {
  try  
 {
 con.Open（）; 
  string  query =（  SELECT tCode， tName FROM tblData ORDER BY tName）; 
 SqlCommand cmd =  new  SqlCommand（query，con）; 
 
 DataTable dt =  new  DataTable（）; 
 dt.Load（cmd.ExecuteReader（））; 
 
  foreach （DataRow drRow  in  dt.Rows）
 { 
  string  empInfo = drRow [  tName ]的ToString（）; 
 cboTests.Items.Add（empInfo）; 
} 
 
  foreach （ string  itm  in  cboTests.Items）
 {
 cboTests.AutoCompleteCustomSource.Add（itm）; 
 
} 
 
 cboTests.DataSource = dt; 
 cboTests.DisplayMember =   tName; 
 cboTests.ValueMember =   tCode; 
 
 
} 
  catch （Exception ex）
 {
 MessageBox.Show（< span class =code-string> 错误： + ex.Message.ToString（））; 
} 
  finally  {con.Close（）; } 
} 
 
  private   void  cboTests_SelectedIndexChanged（ object  sender，EventArgs e）
 {
 
 DataRow selectedDataRow =（（DataRowView）cboTests.SelectedItem）.Row;  ///  这里我收到错误
   int  selectedValue = Convert.ToInt32（selectedDataRow [  tCode]）; 
 
 MessageBox.Show（ 选择的ID为： + selectedValue）; 
} 

解决方案

添加支票，如果 SelectedItem 不为空。



这是最容易检测和修复的案例之一。它只是意味着某些引用类型的某个成员/变量通过使用和它的实例（非静态）成员解除引用，这要求此成员/变量为非null，但实际上它似乎为null。只需在调试器下执行它，它将停止抛出异常的执行。在该行上设置一个断点，重新启动应用程序并再次到达这一点。评估下一行中涉及的所有引用，并查看哪一个为null，而不需要为null。解决这个问题之后，修复代码：要么确保将成员/变量正确初始化为非空引用，要么将其检查为null，如果为null，则执行其他操作。



另请参阅：想要在按钮点击时显示下一条记录。但是如果下一条记录功能的条件对象引用没有设置为对象的实例 [ ^ ]。



代码有不同的问题。异常处理过于本地化。这是你可以捕获所有非特异性异常的UI线程之上，主要面向事件的周期：

的捕获例外 [ ^ ]。



避免转换使用 int.Parse 或 int.TryParse 。避免硬编码立即常量，明确定义常量。



祝你好运，

- SA

I am trying to implement autocomplete combobox from database as in the following code. My objective is to get the value of selected item. Although it is returning the required value but with an error "Object reference not set to an instance of an object". Please help me. Thanks.

private void Form_Load(object sender, EventArgs e)
        {
            cboTests.AutoCompleteSource = AutoCompleteSource.CustomSource;
            cboTests.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
            LoadTest();
            
        }

private void LoadTest()
       {
           try
           {
               con.Open();
               string query = ("SELECT tCode,tName FROM tblData ORDER BY tName");
               SqlCommand cmd = new SqlCommand(query, con);
               
               DataTable dt = new DataTable();
               dt.Load(cmd.ExecuteReader());

               foreach (DataRow drRow in dt.Rows)
               {
                   string empInfo = drRow["tName"].ToString();
                   cboTests.Items.Add(empInfo);
               }

               foreach (string itm in cboTests.Items)
               {
                   cboTests.AutoCompleteCustomSource.Add(itm);

               }

               cboTests.DataSource = dt;
               cboTests.DisplayMember = "tName";
               cboTests.ValueMember = "tCode";
              
               
           }
           catch (Exception ex)
           {
               MessageBox.Show("Error:" + ex.Message.ToString());
           }
           finally { con.Close(); }
       }

private void cboTests_SelectedIndexChanged(object sender, EventArgs e)
       {

           DataRow selectedDataRow = ((DataRowView)cboTests.SelectedItem).Row;          ///Here i am getting the error
           int selectedValue = Convert.ToInt32(selectedDataRow["tCode"]);
 
           MessageBox.Show("Selected Id is: " + selectedValue);
       }

解决方案

Add a check if SelectedItem is not null.

This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[^].

The code has different problems. The exception handling is too local. This is how you can catch all non-specific exception on top of UI thread, in the main event-oriented cycle:
Catching an Exception[^].

Avoid Convert use int.Parse or int.TryParse. Avoid hard-coding of immediate constants, explicitly define constants.

Good luck,

—SA

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